Using the standard sign convention (light travels left to right, distances measured from pole, +ve to right), the MIRROR FORMULA relating object distance $u$, image distance $v$, and focal length $f$ is

1. NCERT §9.2.5 derives the mirror formula from the geometry of paraxial rays reflecting off a spherical mirror. 2. The result: $\dfrac{1}{v} + \dfrac{1}{u} = \dfrac{1}{f}$. 3. Sign convention (Cartesian): u, v, f are all NEGATIVE for real objects, real images and concave mirrors (since they're left of the pole). 4. Distinct from the LENS formula $1/v - 1/u = 1/f$ (different sign — option A is the lens formula, not the mirror one). 5. Numerical example: concave mirror $f = -10$, object at $u = -30$ → $1/v = 1/-10 - 1/-30 = -1/15$, so $v = -15$. Image at $15\,\text{cm}$ in front of mirror. 6. Option C is rearranged but logically equivalent (sign-wise also fine); however the canonical NCERT form is option B. Option D has wrong functional form. _Source: NCERT Class 12 Physics Part 2, Ch 9, §9.2.5 (Mirror Formula, Eq. 9.4), p. 4–5._